The present invention relates to papermaking blankets in general, and to blankets having parallel grooves for use in an extended nip press in particular.
Paper, once formed on the fourdrinier, has a high water content which must be reduced substantially to yield usable paper. One approach to removing this excess water is to run the formed web through an extended nip press apparatus. Such an apparatus has a backing roll which is engaged by a concave press shoe over an extended portion of the roll, hence forming a nip which is considerably extended over the line contact typically found between two opposed rolls. To facilitate movement of the paper web through the extended nip, the web and the press felt are supported by a continuous looped blanket which travels around the press shoe. The water removal capacity of the extended nip press is so great, that it is necessary to provide a multiplicity of circumferential parallel grooves in the press blanket to allow adequate draining of the pressed fluid. Typically the grooves have been cut in a polyurethane surface coating existing alone or formed on an underlying fabric substrate.
However, due to the great pressures applied to the press blanket, on the order of 800 pounds per linear inch, there is a tendency for the portions of the blanket between grooves to collapse and thereby block fluid flow through the grooves. A number of approaches to preventing this collapse have been developed. Higher hardness urethane resists collapse, but is more prone to fracture. Grooved covers for papermaking rolls have been proposed which employ a rubber compound having anisotropic properties provided by reinforcing fibers which restrict collapse in the cross-machine direction, but which allow flexure in the machine direction.
What is needed is an extended nip press blanket having a urethane surface with circumferential grooves which resists collapse and which may be economically fabricated.
The blanket for an extended nip press of this invention is formed in a urethane ribbon casting process. A conventional blanket material, preferably a woven fabric or scrim, is formed into a loop of the desired dimensions and impregnated and coated with a two component urethane resin. As the urethane is applied to the woven substrate, short fibers are introduced into the urethane through a nozzle which aligns the fibers to run in the cross-machine direction. The fibers are introduced in sufficient quantities to provide significant structural reinforcement of the urethane in the cross-machine direction. The coated blanket is then ground to a desired constant thickness and an array of parallel grooves running in the machine direction are milled into the urethane coating. The anisotropic fiber reinforcement provides sufficient modulus to the urethane system to prevent the grooves from collapsing under the applied loads of the extended nip press. The reinforcing fibers may be glass, carbon fiber, synthetic polymer or other structural fiber material.
It is a feature of the present invention to provide a grooved blanket for an extended nip press which will maintain its water channelling capabilities under applied loads.
It is also a feature of the present invention to provide a grooved blanket for an extended nip press which resists barrelling in the grooves.
It is another feature of the present invention to provide a process for forming an extended nip press blanket which has anisotropic reinforcement.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.